Abstract:Molecular sensors are the biosensors made of macrobiomolecules. They are significant tools for real-time imaging of the molecular events in living cells and in vivo. Molecular beacons (MB), fluorescence/bioluminescence resonance energy transfer sensors (FRET/BRET) and molecular fluorescence complementation systems (BiFC, TriFC, etc.) are major types of molecular biosensors for live cell imaging applications. In this paper, the principles and characteristics of the molecular biosensors are described. The biosensors show their unique role in study of biomolecular interactions, localization, movement, and kinetics in living cells. The restrains, challenges, and future prospects of the molecular biosensors are also discussed. Because "Seeing is believing", the molecule biosensors enable us to see the biological process with super high spatio-temporal resolution, and thus to answer the fundamental biological questions.